For a testing of an integrated circuit, etc., a method of using an optical pulse having narrow pulse width as a sampling pulse (for example, disclosed in Japanese Laid-open Patent Application No. 1-110743, U.S. Pat. No. 4,891,580, etc.) has been known as a method for testing an internal node of the integrated circuit, etc. in non-contact and non-destructive state at a high speed.
FIG. 6 is a schematic diagram showing the tip construction of a non-contact type probe of a conventional non-contact type voltage measuring apparatus and the arrangement relationship between the non-contact type probe and a circuit under test.
In FIG. 6, wirings 12 and 13 are formed on the surface of a substrate I1 which constitutes the circuit under test. Here, the wiring whose voltage is actually measured (wiring under test) is represented by a reference numeral 12, and wirings other than the wiring under test 12 are represented by a reference numeral 13.
A non-contact type probe 60 is disposed above the wiring under test 12 in such a manner that a gap is formed between the tip of the probe 60 and the surface of the substrate 11. Electrooptic material (material having electrooptic effect) 61 is provided at the tip portion of the non-contact type probe 60 so as to be supported by a base stage 63, and a reflection film 62 is formed at the tip of the electrooptic material 61. In FIG. 6, a laser beam 31 for detection is also shown.
In the non-contact type probe 60 thus constructed, the polarization variation of the detection laser beam 31 in the electrooptic material 61 due to Pockels effect as described later is dependent on the crystal symmetry and crystal orientation of the electrooptic material 61 and the direction of a fringe electric field supplied from the wiring under test 12. The electrooptic material 61 is formed of lithium niobate (LiNbO3), gallium arsenide (GaAs), zinc telluride (ZnTe) or the like, and in the following embodiment, GaAs is used for the electrooptic material 61.
As well known, GaAs is a compound semiconductor of III-V group. The crystal symmetry of GaAs belongs to cubic system, and thus no birefringence occurs in a state where no electric field is applied to the crystal. However, in a state where an electric field is applied to the crystal, the birefringence in proportion to the applied electric field occurs because it has no body center symmetry. Such a phenomenon is known as a primary electrooptic effect, that is, Pockels effect.
This state is represented by the following fundamental equations for an index ellipsoid with an applied electric field and an electrooptic effect. ##EQU1##
Here, i, j, k represent integers indicating the crystal direction, r.sub.ijk represents an electrooptic coefficient, and n.sub.ij represents index of refraction. For GaAs, values other than three electrooptic coefficients r.sub.41, r.sub.52 and r.sub.63 are equal to zero, and the following equation is satisfied. EQU r.sub.41 =r.sub.52 =r.sub.63 ( 3)
In consideration of these relationships, when the direction of a light incident to GaAs is "3" direction, a phase difference (phase retardation) which is caused by the difference between two principal indices of refraction due to the birefringence induced by the electric field. ##EQU2##
In the equation (4), the integration is carried out over the thickness d of the electrooptic material (in this case, GaAs) along an optical path of an incident light and a reflected light. Therefore, the integration value corresponds to the difference in potential at the light-passing positions on the surface of the crystal and the back surface of the crystal, and is dependent on only an electric field component in the propagating direction of the light. On the basis of this integration value, an electric field component in parallel to the light in the GaAs crystal (longitudinal effect) is detected.
As described above, by detecting the variation in polarization of the detecting laser beam 31 which is caused due to the Pockels effect, the difference in potential between the surface of the electrooptic material 61 having the reflection film 62 and the opposite surface of the electrooptic material 61 supported on base storage 63 is determined. Since this difference in potential is calibrated to represent the voltage of the wiring under test 12 on the assumption that the distance between the reflection surface 62 and the wiring under test 12 is fixed to a predetermined distance, the voltage of the wiring under test 12 is obtained on the basis of the difference in potential.
However, as shown in FIG. 6, when a circuit wiring 13 which is not an object to be measured is disposed adjacently to the wiring under test 12, the testing may suffer the affection of a so-called cross-talk. That is, lines of electric force extending from the wiring under test 12 are easily distorted due to the voltage of the circuit wiring 13 adjacent to the wiring under test 12, so that there occurs a case where the lines of electric force are not vertical (in parallel to the light) in areas other than the extremely adjacent area to the wiring under test 12. That is, the electric field extending from the circuit wiring 13 is penetrated through the reflection surface 62 and intrude into the path of the laser beam in the electrooptic material 12, so that an electric fields other than the electric field to be originally detected is also detected.
The electric field in GaAs is weakened because GaAs has high dielectric constant (about 13). Therefore, a part of the fringe electric field extending from the wiring under test 12 is passed through the electrooptic material 61, and electric fields other than the fringe electric field are concentrated in the gap between the wiring under test 12 and the electrooptic material 61. As a result, the state of the lines of electric force which pass through the electrooptic material 61 are greatly varied in accordance with the variation of the gap between the wiring under test 12 and the electrooptic material 61. Therefore, if there is a control error in the distance between the wiring under test 12 and the electrooptic material 61, this control error appears as a measurement error for the voltage of the wiring under test.
In addition, the lines of electric force which are thrust into the gap between the wiring under test 12 and the electrooptic material 61 are extremely sensitive to a voltage existing adjacent thereto, and the intensity of the electric field in the electrooptic material 61 is varied by the voltage of the adjacent wiring 13.
Consequently, even when the same voltage is actually measured, the integration value of the equation (4) is not necessarily constant in accordance with the state of a circuit under test.
As described above, in the conventional non-contact type voltage measuring apparatus using the non-contact type probe 60 as shown in FIG. 6, the variation of the polarization (birefringence) due to Pockels effect of the detection laser beam 31 which goes and returns through the electrooptic material 61 is not only greatly varied by the voltage of the wiring 13 adjacent to the wiring under test 12 (that is, suffers the affection of a cross-talk), but also suffers the affection of the gap between the wiring under test 12 and the electrooptic material 61, so that there occurs a problem that the measurement precision is lowered.
If the electrooptic material 61 is contacted with the wiring under test 12, the problem that the variation in polarization is dependent on the gap interval is avoidable. However, in this case, the following great advantages of the non-contact measurement are lost: (1) a wiring under test and a circuit suffer no damage, (2) any position can be rapidly scanned for measurement, (3) there occurs no additive effect such as a parasitic capacitance or the like due to the contact, (4) even a circuit wiring which is coated with a passivation film such as semiconductor devices can be measured, etc., and thus it is apparent that the above solution is not realistic.